Various methods that are based on absorption, adsorption, membrane, and cryogenics technologies have been in use for removing environmentally harmful gases such as CO, CO2, SO2 and N2O emitted from, e.g., chemical plants, power plants, and large-scale boilers. The absorption or adsorption method, in particular, has been used for selective separation and removal of relatively low concentrations of noxious gases. However, chemical absorbents or adsorbents used for such purposes tend to deteriorate with the processing time, thereby requiring their replacement on a regular basis. A solid adsorbent, which is normally more resistant to chemical degradation, might be more suitable for such a process. Notwithstanding such apparent advantage, however, use of a liquid absorbent, instead of a solid adsorbent, has been preferred in the purification/separation of large quantities of, e.g., exhaust gas mixture due to the former's relative ease of replacement and handling.
For instance, amine-based compounds have been extensively used as sulfur dioxide (SO2) absorbents. Representatives of such amine-based compounds include triethanol amine disclosed in U.S. Pat. No. 3,904,735, monoethanol amine disclosed in U.S. Pat. No. 4,201,752, and diethanol amine disclosed in U.S. Pat. No. 2,404,854. Use of an amine-based SO2 absorbent typically involves an absorption step wherein SO2 molecules and the absorbent form a chemical bonding, followed by a regeneration step to recover the bound SO2 by way of, e.g., thermal treatment. However, such amine-based SO2 absorbents have severe drawbacks in that the regeneration step must be carried out at a high temperature due to the relatively strong chemical bond between SO2 and the amine, which may cause an irreversible decomposition of the amine, which may, in turn, cause reduction in absorption capacity, corrosion of the absorption apparatus due to the amine or decomposition by-products thereof, and contamination of the recovered SO2 gas by such decomposition by-products.
In order to overcome the afore-mentioned drawbacks, therefore, an ionic liquid-based absorbent has been recently developed. For example, U.S. Pat. Nos. 6,849,774 and 6,623,659, and US Patent Publication No. 2008-0146849 propose the use of an ionic liquid-based absorbent which has negligible volatility and high thermally/chemically stability. Ionic liquid is a polar salt composed of an organic cation moiety and an organic/inorganic anion moiety, and has a high absorption capacity for polar gases such as CO, CO2, SO2, and N2O. Since gas absorption in an ionic liquid depends on the interaction between a gas and the ionic liquid, the absorption capacity for the target gas can be adjusted to a certain extent by tailoring the polarity, acidity, basicity, or nucleophilicity of the ionic liquid by way of varying the types of cation and anion moieties in the ionic liquid.
It has been disclosed [Angew. Chem., Int. Ed., 2004, 43, 2415-2417] that an SO2 absorbent comprising an ionic liquid of 1,1,3,3-tetramethylguanidinium lactate ([TMG]L) absorbs 0.978 mole of SO2 per mole of the ionic liquid. Korean Patent No. 10-0831093 discloses that 1-butyl-3-methylimidazolium chloride ([BMIm]Cl) absorbs 1.68 mole of SO2 per mole of the ionic liquid, while 1-ethyl-3-methylimidazolium ethylsulfate ([EMIm]EtSO4) absorbs 0.92 mole of SO2 per mole of the ionic liquid. Further, Korean Patent Publication No. 2010-0043796 describes that a fluorine-containing ionic liquid, 1-butyl-3-hexafluoropropyl imidazolium trifluoroacetate ([BhFpIm]CF3CO2), which is highly stable against heat and SO2-induced degradation, absorbs 0.48 mole of SO2 per 1 mole of the ionic liquid.
However, ionic liquid absorbents reported in the prior arts still possess several drawbacks. For instance, in case of an ionic liquid absorbent containing fluorinated anion such as tetrafluoroborate (BF4), the fluorine moiety can be easily hydrolyzed to generate hydrogen fluoride, thereby causing the loss of the absorbent. In case of an ionic liquid with Cl− anion, it exists as a solid at a room temperature and, therefore, during an absorption process, it must be maintained at a relatively high temperature so as to keep the absorbent in a liquid state.